Noise filter capacitors are used for eliminating noise emanating from the power lines of various household electric appliances and electronic apparatii and for preventing noise generated in such apparatii from going into the power lines. It is possible for a large rush of current to surge into capacitors, including noise filter capacitors, when lightning strikes. Lest capacitors should be broken or burned out by the surge, standards are established in respective nations to specify severe tests to be performed for capacitors such as noise filter capacitors (hereinafter sometimes referred to as "safety standards" or "standards"). The standards include, UL 1414, CAS C22.2 NO1, VDE 0565-1, IEC 384-14, SEV 1055, JIS C 5151, NEMKO, SEMKO, DEMKO, E1, BS, and the like.
These standards, assuming a large surge, specify that when a voltage that is very high compared to the rated voltage is applied for a certain time, the capacitor shall not become abnormal, burned out or broken.
To conform to these standards, an 11 to 12 .mu.m thick polyester film and an aluminum deposited film with a wide margin width have been used. However, they have the problem that the defective fraction is large since their quality standards conform to a very narrow tolerance range. In other words, the rejection rate in safety voltage tests is large. On the other hand, there is a strong demand for downsizing capacitors, and if the film thickness is reduced for downsizing, the defective fraction increases.